In the following description, publications referenced herein are identified with bracketed numbers (e.g., [1]) which refer to the list of references identified in the List of References following the description. Optical high-voltage sensors often rely on the electro-optic effect (Pockels effect) in crystalline materials such as Bi4Ge3O12 (BGO) [1]. An applied voltage introduces a differential optical phase shift between two orthogonal, linearly polarized light waves propagating through the crystal. This phase shift is proportional to the voltage. At the end of the crystal, the light waves commonly interfere at a polarizer. The resulting light intensity serves as a measure for the phase shift and thus the voltage.
U.S. Pat. No. 4,904,931 [2] and U.S. Pat. No. 5,715,058 [3] disclose a sensor in which the full line voltage (up to several 100 kV) is applied over the length of a single BGO crystal [1]. A method used to retrieve the applied voltage from the resulting modulation pattern is described in [4]. An advantage is that the sensor signal corresponds to the true voltage (that is, the line integral of the electric field along the crystal). However, the electric field strengths at the crystal are very high. In order to obtain sufficient dielectric strength, the crystal is mounted in a hollow high-voltage insulator made of fiber-reinforced epoxy filled with SF6 gas under pressure for electric insulation. The insulator diameter is sufficiently large to keep the field strength in the air outside the insulator below critical limits.
In EP 0 316 635 [5], a sensor is disclosed where the applied voltage is approximated by multiple local electric field measurements using piezoelectric sensing elements such as the ones described in more detail in EP 0 316 619 [6]. With proper choice and orientation of the piezoelectric crystals, only one component of the electric field is measured and thus the sensitivity to external field perturbations is reduced. A similar concept has been described in U.S. Pat. No. 6,140,810 [7]. Here, however, the individual piezoelectric sensing elements are equipped with field steering electrodes and connected with electric conductors such that full integration of the electric field is performed. Dividing the voltage among several crystals reduces the peak electric fields such that a slim insulator is sufficient to provide the required dielectric strength.
U.S. Pat. No. 6,252,388 [8] and U.S. Pat. No. 6,380,725 [9] disclose a voltage sensor which uses several small electro-optical crystals mounted at selected positions along the longitudinal axis of a hollow high-voltage insulator. The crystals measure the electric fields at their locations. The sum of these local field measurements serves as an approximation of the voltage applied to the insulator. Here, the field strengths at a given voltage are significantly lower than with the design of [2] and insulation with nitrogen at atmospheric pressure is sufficient. However, since the sensor does not measure the line integral of the field but derives the signal from the field strengths at a few selected points between ground and high voltage, extra measures (permittivity-shielding or resistive shielding) to stabilize the electric field distribution are necessary to avoid excessive approximation errors [9].
A drawback of the above concepts is the requirement of an expensive high-voltage insulator of large size. The outer dimensions are similar to the ones of corresponding known inductive voltage transformers or capacitive voltage dividers. Thus, the attractiveness of such optical sensors is limited.
Refs. [10] and [11] describe an electro-optical voltage sensor of the type as in [2, 3], but with an electro-optic crystal embedded in silicone. A hollow high-voltage insulator of large size and SF6-gas insulation is thus avoided. As in [7], the voltage may be partitioned on several crystals.
When only a fraction of the total voltage is measured, more compact integrated sensor arrangements can be used. See, for example, U.S. Pat. No. 5,029,273 [12].
Various techniques used to extract the electro-optic phase modulation from the measured signals are known. As described above, the technique used in [4] relies on the applied voltage exceeding the half-wave voltage of the electro-optic crystal. Moreover, a signal at quadrature or some other means to achieve a non-ambiguous output is required. An advantage of this technique is that the light can be guided from the light source to the sensor crystal using standard single-mode (SM) or multi-mode (MM) fibers—i.e. no polarization-maintaining (PM) fibers are needed. The polarizers required to obtain the linear polarization for the measurement can be incorporated into the sensing element. Likewise, the return fiber to the detector can be a non-PM fiber.
For voltages much lower than the crystals' half-wave voltage, another polarimetric technique is published in [13]. This technique is particularly suitable for measurements using local field sensors. These sensors only measure a fraction of the total line voltage much smaller than the voltages measured with full integration.
A technique for the retrieval of the electro-optic phase shifts based on non-reciprocal phase modulation is known from fiber-optic gyroscopes [14] and has also been described for fiber-optic current sensors [15]. It has been adapted for use with piezo-electric and electro-optic voltage sensors [16, 17]. It is particularly suitable for small phase shifts, but in general requires the use of PM fibers for the link between an optical phase modulator—generally located near the light source and detector—and the sensing element positioned in the high voltage insulator.
Another concept is known from high-voltage bushings. There is often a need in high-voltage systems to pass high-voltage conductors through or nearby other conductive parts that are at ground potential (e.g., at power transformers). For this purpose, the high-voltage conductor is contained within a feed-through insulator. The insulator contains several layers of metal foil concentric with the high-voltage conductor and insulated from each other. By appropriately choosing the length of the individual cylinders of metal foils, the distribution of the electric field within and near the bushing can be controlled in such a way that a relatively homogeneous voltage drop from high-voltage to ground potential occurs along the outer surface of the bushing [18, 19].